Phase-shifting masks have recently been found to facilitate fabrication of new generations of integrated circuits and dynamic random access memories (DRAMs). In phase-shifting masks for exposing a photoresist pattern, phase-shift material delays the electromagnetic wave passing through the pattern so it arrives substantially 180.degree. out of phase with the wave passing through the transparent apertured areas of a nonphase-shifting material. (As herein used, the term "nonphase-shifting material" connotes a material that introduces a constant phase shift, and the term "phase-shifting material" connotes one that introduces an additional phase shift due to a difference in thickness or refractive index gradient or a combination of both.) This creates destructive interference between the images of the nonphase-shifting regions and the phase-shifting regions which enhances the contrast of the overall image. It is known that the dark fringe from two closely-spaced parallel dark phase-shifted edges can be darker than the image of a single narrow opaque line and thus provides what has been termed a "darker-than-dark" mask.
The exact conditions under which the two edges merge depend upon the resist processes used for lithography, but generally occur for mask features with k.sub.1 between 0.35 (when the fringe is barely dark enough to print) and 0.6 (when the two edges separate into two distinct dark lines). The width w of a feature as optically imaged is given by ##EQU1## where NA represents the numerical aperture (typically 0.45 for modern i-line steppers) and .lambda. is the wavelength (.lambda.=0.365.mu.n for i-line steppers).
Phase-shifting masks may or may not include a chrome layer. The present invention relates to an improved so-called transparent phase-shifting mask that preferably does not but may include any chrome or other material to create regions that print or project as dark areas.
The most pertinent prior art known to applicants is:
[A] Toh et al., "Chromeless Phase-shifted Masks: A New Approach to Phase-shifting Masks", presented at the Tenth Annual Symposium on Microlithography of BACUS, September 1990.
[B] Watanabe et al., "Sub-quarter-micron Gate Pattern Fabrication Using a Transparent Phase Shifting Mask", published by the Journal of Vacuum Science Technology, Vol. B9, No. 6, November/December 1991, pp. 3172-3175.
Reference [A] describes transparent (i.e., chromeless) phase-shifting lithographic masks that write patterns of contiguous structures from closely-spaced regions of phase-shifting material covering selected areas of a transparent nonphase-shifting material. One embodiment (at the top of FIG. 13) is a mask with two integral orthogonally arranged phase-shifting regions that form a right-angle elbow; but (as shown at the bottom of FIG. 13) the image intensity is nonuniform because the diagonal distance between the inner and outer edges at the junction of the elbow is greater than that between the parallel edges of the straight sections of the elbow. If the diagonal distance is great enough, the images of the phase-shifting regions will not merge at the junction and the bright spot thus created will cause an undesired removal of photoresist at the junction. Another embodiment (shown in FIGS. 14 and 15) depicts a mask layout pattern consisting of a "checkerboard" arrangement of phase-shifting and nonphase-shifting regions in which the phase-shifting regions have contiguous corner contact with those in adjacent rows and columns.
Reference [B] discloses (in FIG. 5) a plurality of spaced parallel regions of phase-shifting material.
Neither of these references teaches nor remotely suggests providing noncontiguous regions of phase-shifting material having respective phase-shifting edges spaced apart at arbitrarily selectable differing angular orientations to provide transparent lithographic masks and methods of fabricating same which (1) can provide images with wider line widths than those presently known to applicants; and which, in addition, (2) can provide improved depth of focus and/or more latitude in times and intensities in the exposure processing window; (3) enable the mask for certain applications to be written with only one exposure; and (4) enable optical inspection techniques without requiring subresolution to check integrity of the masks in a single electron beam write operation.